Polarization is a property of certain waves that they oscillate with more than one orientation. Electromagnetic waves, such as light, exhibit polarization. In a light wave, both the electric field and magnetic field are oscillating, but in different directions. The oscillation of these fields may be in a single direction (linear polarization), or the field may rotate at the optical frequency (circular, or elliptical, polarization). Where the field rotates, the direction of rotation, and thus the specified polarization, may be either clockwise or counter clockwise.
The most common optical materials (such as glass) are isotropic, meaning they simply preserve the polarization of a wave, but do not differentiate between polarization states. Some classes of materials known as birefringent, or optically active, generally modify a wave's polarization, or affect wave propagation passing through the material.